linux-kernel - Re: [PATCH] nextfd(2)

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Date:	Sun, 01 Apr 2012 15:13:31 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Alexey Dobriyan <adobriyan@...il.com>
CC:	akpm@...ux-foundation.org, viro@...iv.linux.org.uk,
	torvalds@...ux-foundation.org, drepper@...il.com,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] nextfd(2)

On 04/01/2012 03:03 PM, H. Peter Anvin wrote:
> On 04/01/2012 05:57 AM, Alexey Dobriyan wrote:
>>
>> * /proc/self/fd is unreliable:
>>   proc may be unconfigured or not mounted at expected place.
>>   Looking at /proc/self/fd requires opening directory
>>   which may not be available due to malicious rlimit drop or ENOMEM situations.
>>   Not opening directory is equivalent to dumb close(2) loop except slower.
>>
> 
> This is really the motivation for this... the real question is how much
> functionality is actually available in the system without /proc mounted,
> and in particular if this particular subcase is worth optimizing ...
> after all, if someone is maliciously setting rlimit, we can just abort
> (if someone can set an rlimit they can also force an abort) or revert to
> the slow path.
> 

A few more observations:

- There is a huge backwards compatibility problem with this for a
substantial transition period; using /proc/self/fd has worked for a very
long time already.

- Your nextfd() system call will require more system calls that the
typical case for reading /proc/self/fd, because each getdents() system
call handles multiple readdir() invocations.

	-hpa


-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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